Properties of electroretinographic intensity-response functions in retinitis pigmentosa

Dark-adapted electroretinogram (ERG) b-wave amplitudes and implicit times were recorded as a function of stimulus luminance for 15 retinitis pigmentosa (RP) patients and 15 normal subjects. B-wave amplitude as a function of log stimulus luminance was fit by non-linear regression with the Naka-Rushton equation, which has 3 independent parameters: The maximum response (Rmax), slope (n) and half-saturation constant (K). B-wave implicit-time as a function of log stimulus luminance was fit by linear regression. Compared to normal, the RP Rmax values were markedly reduced, suggesting response compression; the RP K values were elevated by an average of 0.76 log unit, suggesting relatively small losses in retinal sensitivity. There was no correspondence between Rmax and visual field area for the RP patients (coefficient of correlation = -0.02). All but 2 of the 15 RP patients had normal or shallower-than-normal implicit-time intensity-response functions, indicating that over most of the dynamic range of the ERG, the implicit-times were either normal or faster-than-normal. These results are discussed in terms of possible RP disease mechanisms.